...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The type of each element in the list.
using value_type = std::pair< string_view, param_list >;
The first element of the pair is the extension token, and the second element of the pair is an iterable container holding the extension's name/value parameters.